Platelets are essential elements involved in haemostatic events. Their role in hemostasis is distinguished by two distinct response: (1) adhesion--the interaction of platelets with subendothelial connective tissue and (2) aggregation--platelet to platelet cohesion. Abnormal platelet function may contribute to a variety of pathophysiological conditions including thrombosis, atherosclerosis, myocardial infarction, stroke and pulmonary embolism.
Inhibitors of platelet aggregation are important in the prevention and treatment of, inter alia, cardiovascular and cerebrovascular diseases. The use of such inhibitors helps in the prevention of unwanted clots which could have detrimental or debilitating effects on patients with high blood pressure, atherosclerosis and other related diseases. These inhibitors are either proteinaceous or non-proteinaceous in nature and inhibit platelet aggregation by a variety of mechanisms. For example, the enzyme inhibitors such as ADPase, fibrinogenase and phospholipase A.sub.2 and the nonenzymatic proteins such as disintergrins and mambin exert their inhibitory effect by various mechanisms. These inhibitors exert their antiplatelet actions by various mechanisms. In the case of nonenzymatic proteins the mechanism appears to by simpler. Mambin and disintegrins that contain Arg-Gly-Asp (RGD) sequence exert their action by competively blocking fibrinogen binding to platelet glycoprotein IIb/IIIA. Ca.sup.2+ -dependent type lectin-related proteins exhibit the effects on platelet agglutination and aggregation by specifically binding to platelet glycoprotein Ib. Among enzymes, some proteinases have been studied for their antiplatelet effects. The inhibitory activity of fibrinogenase was initially thought to be due to fibrinogen degradation, since fibrinogen is involved in the final stages of platelet aggregation. Subsequent studies have shown that fibrinogen is involved in the final stages of platelet aggregation. Recent studies have indicated that some of the metalloproteinases inhibit platelet aggregation by cleaving glycoprotein lb which is a receptor for von-Willebrand factor. The ADPase inhibit platelet aggregation by hydrolysis of ADP to form AMP, an inhibitor of platelet aggregation. Thus these enzymes physically destroy either ligand and/or receptor due to their inherent enzymatic activity. Accordingly, different inhibitors have distinct advantages and therefore new sources of anti-clotting agents are constantly sought.
The increasing demand for new pharmaceutical agents has led the pharmaceutical industry to consider molecules found in the natural environment. Accordingly, much effort is being spent on screening aquatic environments, riverbeds, coral, plants, microorganisms and higher animals for potentially useful molecules. The screening process is often referred to as "natural product screening".
In work leading up to the present invention, the inventors have studied snake venom from the common Death Adder (Acanthophis antarcticus) and have identified and sequenced a novel protein molecule capable of inhibiting platelet aggregation.